This invention relates to an electrical filter of the type employed to attenuate harmonic spurs prior to radio transmission. More particularly, this invention relates to a microstrip-type filter based upon a substrate that includes a first dielectric material to produce an optimum capacitive coupling between a radial pad for attenuation of the harmonic spurs and a second dielectric material underlying a transmission line to provide a distinct capacitive coupling with regard to the primary signal.
In a high power radio transmitter of the type employed in a base station of a communication system, an electrical signal is passed through a filter en route to the antenna for broadcast. The filter passes the primary signal of the desired broadcast frequency, while removing signals at higher frequencies, most notably signals at harmonic frequencies, referred to as harmonic spurs. For this purpose, it is desired to minimize insertion loss of the primary signal while maximizing attenuation of the harmonic spurs.
A conventional high power filter comprises a wire transmission line along a central axis of a cylindrical metal housing. The transmission line is connected to a series of circular metal plates that perpendicularly intersect the line. The plates are circumferentially spaced apart from the housing, which is grounded, by a dielectric material to create a capacitive coupling effective to shunt the harmonic spurs. Such filters have been generally satisfactory for suppressing harmonic spurs, but are massive in construction and expensive.
Microstrip filters are known that comprise a transmission line and intersecting pads affixed to a surface of a dielectric board opposite a ground plate. The resonator pads are capacitively coupled to the ground plate and sized and located along the transmission line to attenuate the harmonic spurs. Microstrip filters have been generally suitable for low power transmissions, but suffer from a high insertion loss that reduces the power of the primary signal and is not desirable for high power transmissions.